Polystyrene is a high molecular weight linear polymer produced by polymerizing styrene. When made from only the styrene monomer, the resulting polymer has a glass transition temperature of about 100.degree. C. and is brittle, showing very poor strength (impact strength, elongation to rupture and dart-drop impact strength) characteristics. The strength characteristics can be improved by incorporating rubber modifiers, such as butadiene rubber. When a rubber is incorporated, the product is called "HIPS" for High Impact Polystyrene.
HIPS usually contains about 5 to 15 wt. % polybutadiene or styrene-butadiene copolymer rubber. However, the presence of polystyrene occlusions within the rubber phase typically results in about 10-40% volume fraction of the reinforcing rubber phase. When polymerizing styrene in the presence of polybutadiene, polybutadiene grafts with polystyrene. There is a phase inversion period as the polymerization progresses. The polybutadiene-styrene phase is the continuous phase during the earlier part of the polymerization, with polystyrene dispersed in the polybutadiene-styrene phase. After progressively more polystyrene is formed, there is a phase inversion period and styrene-polystyrene eventually becomes the continuous phase, with polybutadiene dispersed therein, as explained by Freeguard, Br. Polym. J., vol. 6, page 205, 1974.
Whereas various rubbers, such as polybutadiene, have been used to modify polystyrene to improve the strength of polystyrene, polyphenylene oxide has been used to raise the softening point of polystyrene.
Polyphenylene oxide and the like are sometimes referred to as polyphenylene ethers, abbreviated as PPE, and are described in numerous publications, including an article by D. M. White titled "Poly(phenylene oxide)s" at chapter 28, pages 473-481 of Comprehensive Polymer Science, vol. 5, Pergamon Press, 1989, and including U.S. Pat. Nos. 3,306,874; 3,306,875; 3,257,357 and 3,257,358. PPE can be made to grades of sufficiently high molecular weights. Furthermore, high temperatures are necessary for processing PPE resins because of a high softening point in a range of about 210.degree. C. to 280.degree. C.
The incorporation of PPE in polystyrene is typically done by blending, such as blending PPE with polystyrene in an extruder at high temperatures to form PPE and polystyrene blends. For instance, European Patent 121,974 to Van der Meer et al. discloses in Example II the blending of PPE with polystyrene (PS) by heating blends of PPE with PS followed by extrusion. The PS used included rubber-modified polystyrene, and/or styrene-butadiene block copolymer (unsaturated), and/or styrene-butadiene block copolymer (saturated), and various additives. European Patent 168,566 and U.S. Pat. No. 4,423,189 also disclose blends of PPE and PS.
U.S. Pat. No. 3,664,977 to Nakanishi et al. discloses a combination of PPE and HIPS where the PPE is added in situ, that is, during the polymerization reaction to form HIPS type polystyrene from styrene monomer and rubber, as opposed to blending PPE with preformed polystyrene in an extrusion operation.
According to the Nakanishi patent, the PPE is added after phase inversion, in particular, between the time immediately after phase inversion up to the time the total solids level reaches 40%, and preferably when the solids level is 15-35%. Also, the Nakanishi patent states at col. 2, line 38, that the PPE was added in the form of a styrene solution of PPE. Further, at col. 3, line 7, the Nakanishi patent states that, in their process, a considerable amount of PPE is incorporated into the rubber particle portion of the HIPS product.
U.S. Pat. No. 5,660,776 to Wooden et al. is another reference where PPE is added during the polymerization process in the form of a solution of PPE in styrene. The Wooden et al. patent is directed to so-called GPPS (general purpose polystyrene), or crystal polystyrene, as opposed to HIPS. In the Wooden et al. patent, the feed PPE for the process is a solution of PPE in styrene. The solution is obtained by dissolving PPE in styrene. Thus, Wooden states that he provides a process to produce a solution of a styrenic polymer containing up to 15 weight % of polyphenylene ether comprising:
(a) dissolving a polyphenylene ether polymer in one or more of the monomers for said styrene polymer; PA1 (b) feeding the solution obtained in (a) above together with up to 0.05 weight % of one or more organic free radical initiators and from 0 to 10 weight % of an organic diluent and additional monomer to produce a concentration of polyphenylene ether in monomer of up to 15 weight % to one or more reactors; PA1 (c) subjecting said solution to heat for a time sufficient to polymerize at least 70 weight % of the monomers; PA1 (d) subjecting said polymerized solution of styrenic polymer containing polyphenylene ether to temperature and pressure conditions to devolatilize unpolymerized monomer and diluent; and PA1 (e) extruding said solution of styrenic polymer containing polyphenylene ether as strands and cooling said strands and chopping said strands into pellets. PA1 (a) polymerizing styrene in the presence of a rubber to a point past phase inversion, preferably far beyond the phase inversion point, in a first reaction zone to obtain a first reaction mix; PA1 (b) passing the first reaction mix to a second reaction zone wherein the polymerization of styrene is continued; PA1 (c) adding a slurry of PPE in styrene, containing at least 15 wt. % PPE, to the first reaction mix in the second reaction zone; and PA1 (d) further polymerizing styrene in the slurry of PPE in styrene with the first reaction mix in the second reaction zone. PA1 (a) polymerizing styrene in the presence of a rubber to a point just before phase inversion in a first reactor to obtain a first reaction mixture; PA1 (b) passing the first reaction mixture to a second reactor and continuing styrene polymerization therein to a point where the total polymer solids level is greater than 40% of the total reaction mixture by weight and wherein the rubber phase is a dispersed phase as rubber particles and thereby obtaining a second reaction mixture; PA1 (c) passing the second reaction mixture to a third reactor wherein the polymerization of styrene is continued; PA1 (d) dispersing PPE in styrene to form a PPE-styrene slurry containing at least 15 wt % of PPE; PA1 (e) adding the PPE slurry to the third reactor and reacting styrene monomer to polystyrene in the presence of PPE therein to obtain a third reaction mixture; and PA1 (f) passing the third reaction mixture to a fourth reactor and therein substantially completing the polymerization reaction.
In referring to the Nakanishi patent, Wooden states that the Nakanishi U.S. Pat. No. 3,664,977 reference teaches blending polyphenylene ether into the bulk polymerization of high impact polystyrene (HIPS) between a point after rubber phase inversion (e.g., after the point when the phase volume of rubber in styrene is about equal to the phase volume of polystyrene in styrene) to a point when the total polymer concentration has become 40%.